I did something similar using I2C and an 8-bit I/O expander chip. Also took Limor Fried's LiquidCrystal library and generalized it to handle a couple of different connection schemes (including the original 4-/8-pin parallel mode).

If anyone is interested in using the reworked library as base for a general-purpose LCD library which can tie into this SPI connection mode as well, feel free to adapt/modify/extend the code - it can be found at http://news.jeelabs.org/2009/09/26/generalized-liquidcrystal-library/